FlightGear - Flight Simulator FGAddon

Citation: load model correctly over multiplayer

continue work started in r3149.

787-8: multiplayer bug fixes.

* Do not pollute the property tree of the local aircraft: use only relative property
names in animations.
* Disable the shadow for remote 787-8s as they do not transmit their
/position/altitude-agl-ft by default and the shadow is only minor eye candy.

Addons/*/addon-metadata.xml: new header with meta section; explicit node typing where not already done

LS4: Improve sounds, improve cockpit

777: bug fix: animate the gear of the remote 777 with its own properties,
not those of the local aircraft

Citation: load model correctly over multiplayer

Several aspects of the Citation II would not load correctly when all the following
conditions were met simultaneously:

* the low-poly AI model in /usr/share/games/flightgear/AI/Aircraft/Citation was
disabled (e.g. directory renamed).
* the local aircraft was not a Citation II.
* A remote Citation II appeared in range.

All XML file paths must start with Aircraft/Citation.

ATCChatter and Skeleton: use explicit node typing in the metadata file, and more spacing

ATCChatter: update README (much simpler with --addon)

Addons: the metadata file now supports short and long descriptions

ATCChatter: fill in description field in metadata file